Ceramic arc tubes of materials such as polycrystalline alumina with one or more additives to control grain size have been used as the discharge vessel or arc tube for high-pressure sodium lamps. Recently, such arc tubes have been employed with metal halide lamps. See, for example, U.S. Pat. No. 5,424,609. Such arc tubes have comprised 5 piece structures including a cylindrical body, a pair of end closures or buttons, and a pair of electrode receiving rods or capillary tubes sealed to the buttons. Manufacture of such arc tubes required extrusion or pressing of the components, as well as multiple assembly and heat treatment steps that increased handling and, therefore, cost.
Additionally, three piece structures have been proposed. In U.S. Pat. No. 4,766,347 there is shown an arc tube having only a ceramic body with electrode receiving rods sealed directly therein. U.S. Pat. No. 5,426,343 discloses a three-piece structure wherein a sealing button has an extending electrode rod receiving member integral therewith.
All of these approaches require extra heating and handling steps.